ABSTRACT: Utility of Ichnology in Sequence Stratigraphic Studies: Examples from Alabama Paleogene

Charles E. Savrda, Richard A. Huchison

When used in conjunction with other sedimentological evidence, ichnofossils are useful indicators of paleoenvironmental parameters such as physical energy, sedimentation rate, substrate consistency, and bottom-water and/or pore-water chemistry. Because these parameters are controlled in part by sea level dynamics as well as absolute water depth, ichnofossils may be of greater utility in sequence stratigraphic studies than previously recognized. Paleogene strata of the Alabama coastal plain record both spatial and temporal changes in paleobathymetry; lateral facies relationships generally reflect increasing water depth westward across the state, whereas vertical facies changes reflect the stacking of discrete depositional sequences, each of which represents a major eustati cycle. A study of several of these sequences at various localities across the state has been initiated in order to systematically assess the composition and preservational state of trace-fossil assemblages within and across component facies-independent systems tracts and their bounding surfaces (i.e., unconformities, transgressive surfaces, and surfaces of maximum starvation). Thus far, results obtained from studies of two depositional sequences within the lower Paleocene Clayton and Porters Creek formations indicate that trace-fossil assemblages do respond in a manner that is commensurate with predicted sea level mediated changes in environmental parameters. Continued intra- and intersequence studies of trace fossils at the systems-tract level will (1) provide an independent test of se uence stratigraphic concepts and interpreted sea level histories, (2) enhance our understanding of trace fossils as paleoenvironmental indicators, and (3) form the basis for ichnological models useful for recognizing the eustasy signal in other coastal-plain sequences.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990